A functional systems approach to understanding verbal-motor integration in individuals with Down syndrome

Abstract

Introduction Over the last ten years our laboratories at McMaster Univer-sity, and more recently, Simon Fraser University and the University of Alberta, have been actively engaged in re-search concerned with cerebral organization in adults and adolescents with Down syndrome. Our goal has been to determine how patterns of brain organization unique to Down syndrome may contribute to both the specific and general information processing capabilities of individuals with Down syndrome. Much of what we know about brain-behaviour relations in general comes from clinical studies involving people who have suffered a stroke or a head injury which has damaged a localized area of the cerebral cortex. Since the late 1800s, it has been recognized that in most right-handed people the left cerebral hemisphere plays a special role in speech and language. This is because left hemisphere brain damage is much more likely to result in expressive (Broca, 1865) and receptive (Wernicke, 1874) speech and language problems (aphasia) than comparable right hemisphere damage. The left hemisphere also appears to be specialized for the organization and control of voluntary movement, including speech movements. Thus, left hemisphere damage is more likely to result in motor learning and motor control problems (i.e., apraxia; see Roy, 1985 for a review). Although left hemisphere specialization for speech, lan-guage and motor control was apparent to clinicians over 100 years ago, the left hemisphere is no longer referred to as the " dominant " hemisphere by most neuropsychologists. Stud-ies examining persons with right hemisphere brain damage indicate that these individuals are more likely to suffer deficits in tasks that require the perception of the spatial relations between objects in the environment (Jackson, 1958), object recognition (De Renzi, 1968) and selective attention (Heilman and Watson, 1977). It also appears that the right cerebral hemisphere may be involved in the per-ception and expression of emotion (Ley and Bryden, 1979). Thus, the right hemisphere also appears to regulate a number of important functions. In addition to the clinical investigations involving brain-injured people, experimental neuropsychologists have de-veloped a number of noninvasive techniques for examining brain-behaviour relations in the intact brain. Our initial interest in Down syndrome developed as a result of several studies that employed a procedure referred to as dichotic listening. The dichotic listening paradigm is a noninvasive method for examining cerebral specialization for speech perception. Typically, participants are presented with pairs of letters, digits, or words simultaneously to the right and left ears through headphones. In what is termed a " free recall " situation, the participants are asked to report every sound they hear. Alternatively, a " selective listening " procedure may be employed in which the participant is asked to report sounds from one or the other ear. Regardless of the proce-dure, most right-handed children and adults correctly report more right ear items than left ear items. Because of the contralateral or crossed nature of the major auditory path-ways, this right ear advantage for the perception of speech sounds has been taken to reflect left hemisphere speciali-zation for speech and auditory language function. In this paper we present the background, development and application of a functional systems approach to understanding verbal-motor integration characteristic of persons with Down syndrome. Based on our initial work utilising noninvasive, neuropsychological procedures, we have forwarded a specific model of brain-behaviour relations in persons with Down syndrome. The crucial characteristic of the model is the proposed functional disconnection of brain areas responsible for speech perception and movement organisation. In addition to describing the model, we summarize our recent work designed to test, refine, and extend it.